OFDM modulation consists of distributing data with duration Tu (called the useful symbol time) in the time-frequency space on a plurality of independently modulated carrier frequencies, for example in QPSK or QAM. OFDM thus decomposes the channel into cells along the time axis 11 and the frequency axis 12 as shown in FIG. 1. Each of the carriers is orthogonal to the previous carrier.
The channel with predetermined length 13 is thus composed of a sequence of frequency sub-bands 14 and a sequence of time segments 15.
A dedicated carrier is assigned to each frequency/time cell. Therefore, information to be transported will be distributed on all these carriers, each modulated at low flow for example, by a QPSK or QAM type modulation. An OFDM symbol includes all information carried by all carriers at time t.
This modulation technique is particularly efficient in situations in which multi-paths are encountered. As shown in FIG. 2 that presents a set of OFDM symbols 21, the same sequence of symbols arriving at a receiver by two different paths is like the same information arriving at two different and additive instants. These echoes cause two types of defects:                intra symbol interference: addition of a symbol with itself slightly out-of-phase;        inter symbol interference: addition of a symbol with the next symbol plus the previous symbol slightly out of phase.        
A “dead” zone called the guard interval 22 is inserted between each transmitted symbol, the duration 23 of which is chosen to be sufficiently large with respect to spreading of echoes. These precautions will limit inter symbol interference (which is absorbed by the guard interval).
On reception, carriers are also affected by either an attenuation (destructive echoes) or amplification (constructive echoes) and/or phase rotation.
Pilot synchronisation carriers (often with an amplitude greater than useful data carriers) are inserted to calculate the channel transfer function and thus equalize the signal before demodulation. The value and location of these pilots in the time/frequency space are predefined and known to the receivers.
After interpolation in time and in frequency, a more or less relevant estimate of the channel response is obtained as a function of the number of reference pilots and their distribution in the time/frequency domain.
OFDM modulation is increasingly used in digital broadcasting because it is very well adapted to variations in the radio channel:echoes and Doppler. Engineers firstly study the characteristics of the radio channel that vary as a function of the emission frequency, the signal pass-band, and also for digital radio in the AM (DRM) bands, different propagation conditions between day and night and solar cycles, so as to choose the best adapted OFDM structure.
Receivers used for OFDM demodulation essentially use the channel response calculated from reference pilots. Therefore, the accuracy of this estimate depends on the proportion of reference pilots inserted in OFDM symbols.
A common phase error correction algorithm is known, but the treated error corresponds to the relative error between two successive OFDM symbols, the objective then being to correct phase errors due to defects in oscillators used in the receivers.
However, fast channel variations are observed, particularly for DRM and particularly when travelling in cars, which can cause temporary loss of service (partial or total).
In particular, known techniques for correcting phase variations between two successive OFDM symbols are based on calculation of the common phase error by differentiation between two successive symbols. Therefore, this correction is done before the channel estimate. For example, this is the case for the solution proposed by France Telecom and Telediffusion de France (TDF) in their French patent No. FR 2 768 278. However, this may not be sufficient, particularly in the case of DRM.